1. Field of the Invention
This invention relates to a spark plug and production thereof.
2. Description of the Related Art
A spark plug used for ignition of an internal engine of automobiles, etc. generally comprises a metal shell to which a ground electrode is fixed, an insulator made of alumina ceramics, etc. which is disposed inside the metal shell, and a center electrode which is disposed inside the insulator. The insulator projects from the rear opening of the metal shell in the axial direction. A spark plug terminal (hereinafter xe2x80x9cterminalxe2x80x9d) is inserted into the projecting part of the insulator and connected to the center electrode via a conductive glass seal layer, which is formed by a glass sealing technique, a resistor, and the like. A high voltage is applied to the terminal to cause a spark over the gap between the ground electrode and the center electrode.
Under some combined conditions, for example, at an increased spark plug temperature and an increased environmental humidity, it may happen that high voltage application fails to cause a spark over the gap but, instead, a discharge called a flashover occurs between the terminal and the metal shell, going round the projecting insulator. Primarily for the purpose of avoiding flashovers, most of commonly used spark plugs have a glaze layer on the surface of the insulator. A glaze layer also serves to smoothen the insulator surface thereby preventing contamination and to enhance the chemical or mechanical strength of the insulator.
Lead glazes have been applied to alumina ceramics as an insulator. A lead glaze is silicate glass compounded with a relatively large amount of PbO to have a lowered softening point. In recent years, however, with a globally increasing concern about environmental conservation, glazes containing lead have been losing acceptance. In the automobile industry, for instance, where spark plugs find a huge demand, it has been a subject of study to phase out lead grazes in the future, taking into consideration the adverse influencs of waste spark plugs on the environment.
Borosilicate glass- or alkali borosilicate glass-based glazes have been studied as a substitute for the conventional lead glazes, but they have their own disadvantages, such as a high glass transition point or an insufficient insulation resistance. To address this problem, JP-A-11-43351 proposes a leadless glaze composition having an adjusted Zn component content, etc. to improve glass stability without increasing the viscosity (i.e., without reducing flowability) and JP-A-11-106234 discloses a leadless glaze composition which contains a combination of two or more alkali components to improve insulation resistance.
A glaze formed on an insulator of a spark plug is more apt to rise in temperature than on general insulating porcelain because, for one thing, the spark plug is used as fitted into an engine block of a vehicle. Further, in recent years the voltage applied to a spark plug has been increasing with advancing performance of engines. For these, a glaze for this use has been required to have insulation performance withstanding severer conditions of use.
In the light of these circumstances, the glaze composition disclosed in JP-A-11-106234 supra is not always satisfactory in high-temperature insulating performance, particularly the performance as evaluated as a glaze layer formed on an insulator in a spark plug (e.g., anti-flashover properties).
Both the compositions reported in JP-A-11-43351 and JP-A-11-106234, especially the former, have a relatively high Zn component content (10 to 30 mol %). According to the present inventors"" researches, it has been revealed that too high a Zn component content tends to make it difficult to obtain a smooth glazed surface. This tendency is conspicuous when firing is carried out in an atmosphere containing much steam as in a gas-firing furnace.
An object of the present invention is to provide a spark plug having a glaze layer on the insulator thereof, wherein the glaze layer has a reduced Pb content, is capable of being fired at a relatively low temperature, exhibits excellent insulation properties, and can have a smooth surface.
The above object is accomplished by the present invention.
The present invention provides, in its first aspect, a spark plug comprising a center electrode, a metal shell, and an alumina ceramic insulator disposed between the center electrode and the metal shell, at least part of the surface of the insulator being coated with a glaze layer comprising oxides, wherein the glaze of the glaze layer comprises:
1 mol % or less, in terms of PbO, of a lead component,
25 to 60 mol %, in terms of SiO2, of an Si component,
10 to 40 mol %, in terms of B2O3, of a B component,
0.5 to 9.5 mol %, in terms of ZnO, of a Zn component,
5 to 25 mol %, in terms of BaO, of a Ba component,
the total content of the Si component, the B component, the Zn component, and the Ba component being 60 to 98 mol % in terms of the respective oxides,
the total content of the Zn component and the Ba compound being 9 to 30 mol % in terms of the respective oxides, and
2 to 15 mol % of at least one alkali metal component selected from an Na component, a K component, and an Li component in terms of Na2O, K2O or Li2O.
From the environmental consideration it is a premise of the present invention that the Pb component content in the glaze be 1.0 mol % or less in term of PbO. This premise applies to not only the above-described first aspect but the second and the third aspects of the present invention hereinafter described. A glaze with its Pb component content reduced to this level will hereinafter be referred to as a leadless glaze. When a Pb component is present in a glaze in the form of an ion of lower valency (e.g., Pb2+), it can be oxidized to an ion of higher valency (e.g., Pb3+) by a corona discharge from the glaze layer surface, etc. If this happens, the insulating properties of the glaze layer are reduced, which can result in a flashover phenomenon. From this viewpoint, too, the limited Pb content is beneficial. A preferred Pb content is 0.1 mol % or less. It is the most preferred for the glaze to contain substantially no Pb, except a trace amount of lead unavoiodably incorporated together with raw materials.
The glaze used in the first aspect of the present invention has a specifically designed composition for securing insulating properties, optimizing the firing temperature, and improving the firing finish while reducing the Pb content. A Pb component in a conventional glaze has played an important role in softening point adjustment. That is, a Pb component serves to lower the softening point of a glaze moderately to secure flowability in application. In a leadless glaze, it is a B component (B2O3) and an alkali metal component that take part in softening point adjustment. The present inventors have proved that there is a specific range for a B component content that is suited to improve firing finish (specifically a range of from 10 to 40 mol % in terms of B2O3). Particularly when firing is carried out in an atmosphere containing relatively much steam as in a gas-firing furnace, it is very advantageous to limit the B component content within this range.
To limit the total content of the alkali metal component as well as the B component content is effective in facilitating formation of a glaze layer with a uniform thickness and few defects such as seeds or bubbles. For example, in preparing a glaze slurry of a mixed powder, the alkali metal component and the B component, if present in excessive amounts, dissolve in the dispersing medium such as water to increase the viscosity of the slurry. If the visocisty of the slurry extremely increases (e.g., exceeds 1000 mPaxc2x7s), it is difficult to form a coating layer having a uniform thickness, and the possibility of air bubbles entrapment increases. A proper selection of the alkali metal component content and the B component content within the respective specific ranges makes it possible to easily prepare a glaze slurry having a low viscosity and satisfactory flowability thereby to form a glaze layer of uniform thickness and with few defects.
A glaze composition with a reduced B component content would have an increased difference in linear expansion coefficient (i.e., linear expansion coefficient) from the alumina ceramic insulator and also would have a raised softening point to have reduced flowability on firing. This can be compensated for, in the first aspect of the invention, by adding a Zn component, a Ba component, and an alkali metal component. While a Zn component is effective in improving the linear expansion coefficient, the present inventors have found that addition of too much Zn tends to impair transparency to cause another appearance defect. Hence, the Zn component content is limited to a range of from 0.5 to 9.5 mol %, which is lower than in conventional glaze compositions, to avoid such a defect.
The grounds for limiting the range of the content of each component constituting the glaze layer in the first aspect of the invention are described below in detail. If the Si component content is less than 25 mol %, the glaze has too large a linear expansion coefficient and readily suffers from such defects as crazing, resulting in a failure to secure satisfactory glaze finish, the object of the first aspect. If, on the other hand, the Si component content exceeds 60 mol %, the glaze has too high a softening point, resulting in a defective appearance due to insufficient melting. A preferred Si component content is 35 to 55 mol %.
If the B component content is less than 10 mol %, the softening point of the glaze increases to make firing difficult. If it is more than 40 mol %, the glaze layer tends to suffer from crimping and, while dependent on the contents of other components, sometimes has such problems as devitrification, reduction in insulating properties, and missmatching of expansion coefficient with the substrate insulator. A preferred B component content is 20 to 30 mol %.
With a Zn component content of smaller than 0.5 mol %, the glaze has too large a linear expansion coefficient, tending to suffer from defects, such as cracking and peeling. Since a Zn component is also effective in reducing the softening point of the glaze, its shortage leads to an increased softening point, which may result in insufficient glaze firing. Where the content of the Zn component is more than 9.5 mol %, devitrification can occur to make the glaze layer opaque, and the insulating properties of the glaze layer tend to be insufficient. A preferred Zn component content is 3 to 7 mol %.
A Ba component is contributory to improvement in insulating properties and also effective in improving durability (water resistance) and strength. When the Ba component content is less than 5 mol %, the glaze has reduced insulating properties, which may lead to insufficient anti-flashover properties. If the Ba component exceeds 25 mol %, the softening point tends to become too high to carry out firing, and the glaze has too large a linear expansion coefficient, tending to suffer from defects such as crazing. A preferred Ba component content is 5 to 15 mol %. A part of or the whole of the Ba component can be replaced with an Sr component. In this case, a further improvement on impact resistance of the glaze layer can sometimes result. Depending on raw materials used, a Ba component or an Sr component sometimes exists in the glaze in a form other than their oxide form. For example, in using BaSO4 as a Ba source, a sulfur component may remain in the glaze. The sulfur component tends to be localized near the surface of the glaze layer to reduce the surface tension of the molten glaze, which is effective to increase the surface smoothness of the resulting glaze layer.
The total content of the Zn component and the Ba component should range from 9 to 30 mol %. If this total is smaller than 9 mol %, the glaze may have too high a softening point and be difficult to fire. Where the total content is more than 30 mol %, the glaze is liable to devitrification. A preferred total content of the Zn component and the Ba component is 10 to 20 mol %.
The alkali metal component serves to lower the softening point of a glaze. If the alkali metal component content is less than 2 mol %, the glaze will have an increased softening point, which tends to be too high to conduct firing. If it exceeds 15 mol %, the glaze tends to have reduced insulating properties, resulting in insufficient anti-flashover properties. A preferred alkali metal component content is 3 to 10 mol %.
It is preferred to use a combination of two kinds selected from an Na component, a K component, and an Li component as the alkali metal component, which is effective in suppressing reduction in insulating properties. This means that the alkali metal content is allowed to increase while minimizing reduction in insulating properties. As a result, the two objectsxe2x80x94to secure anti-flashover properties and to lower the firing temperaturexe2x80x94can be accomplished at a time. It is possible to add as a third component other alkali metal component(s) in such an amount that does not impair the above-described effect of combined alkali metal components in suppressing reduction of insulating properties. In order to minimize the reduction in insulating properties, the amount of each alkali metal component to be added is desirably 5 mol % or less.
In the first (and also the second) aspect of the present invention, the total content of the Si component, B component, Zn component and Ba component, which are main and essential components of the glaze, is 60 to 98 mol %. When this total exceeds 98 mol %, the glaze tends to have too high a softening point to fire. If the total is less than 60 mol %, it is difficult to adjust the softening point and the linear expansion coefficient while securing insulating properties. A preferred total content of the first and the second components is 70 to 95 mol %.
The present invention further provides, in its second aspect, a spark plug having the same structure as in the first aspect, except that the glaze layer comprises:
1 mol % or less, in terms of PbO, of a lead component,
25 to 60 mol %, in terms of SiO2, of an Si component,
10 to 40 mol %, in terms of B2O3, of a B component,
1.5 to 20 mol %, in terms of ZnO, of a Zn component,
5 to 25 mol %, in terms of BaO, of a Ba component,
the total content of the Si component, the B component, the zn component, and the Ba component being 60 to 98 mol % in terms of the respective oxides,
the total content of the Zn component and the Ba component being 9 to 30 mol % in terms of the respective oxides,
the content of the B component, taken as NB2O3 (mol %), the content of the Zn component, taken as NZnO (mol %), and the content of the Ba component, taken as NBaO (mol %), in terms of the respective oxides, satisfying the relationships: NB2O3 greater than NZnO and NBaO greater than NZnO, and
2 to 15 mol % of at least one alkali metal component selected from an Na component, a K component, and an Li component in terms of Na2O, K2O or Li2O.
In the second aspect of the invention, the Zn component content is increased over that of the first aspect to a range of from 1.5 to 20 mol %, the B component content (NB2O3) is greater than the Zn component content (NZnO), and the Ba component content (NBaO) is greater than the Zn component content. As a result, the glaze composition achieves both accelerated vitrification and further improvement on insulating properties. That is, even though the Zn component content is higher than the upper limit specified in the first aspect (i.e., 9.5 mol %), it is possible to secure satisfactory insulating properties while suppressing appearance defects due to devitrification. In order to enhance the above effects, it is desirable that NB2O3 greater than NBaO greater than NZnO.
The glaze can contain other alkaline earth metal components in addition to the Ba component. This also applies to the glaze layer of the first aspect. In particular, a Ca component and an Sr component is the most effective next to the Ba component or the Zn component in improving the insulating properties of the glaze layer. When the Zn component is used in a relatively large amount, particularly in an amount of 10 mol % or more, it is preferred from the standpoint of transparency and softening point that the glaze comprises an alkaline earth metal R component, wherein R is at least one of Ca, Sr, and Ba, at a content NRO (mol %) of more than 10 mol % in terms of the respective oxides RO and that the content RNO and the content of the zinc component in terms of ZnO, taken as NZnO (mol %), satisfy the relationship: NZnO/(NRO+NZnO)xe2x89xa60.4. In order to enhance the effect on adjustment of the linear expansion coefficient and thereby to further improve the appearance of the glaze layer, it is still preferred that NZnO/(NRO+NZnO) be 0.1 or greater. The Sr component is also effective to improve the impact resistance of the glazed insulator. Here again, part of or the whole of the Ba component can be displaced with the Sr component.
The grounds for limiting the range of the content of each component constituting the glaze in the second aspect of the invention are the same as in the first aspect, except for the Zn component. The lower limit of the Zn component content is 1.5 mol %, slightly higher than that in the first aspect, so as to keep the linear expansion coefficient of the glaze on a proper level when the B component content or the Ba component content is larger than the Zn component content.
If the Zn component content exceeds 20 mol %, it is difficult to prevent devitrification even with an increase in B or Ba component being taken into consideration. A preferred Zn component content is 3 to 9.5 mol %.
The present invention further provides in its third aspect a spark plug comprising a center electrode, a metal shell, and an alumina ceramic insulator disposed between the center electrode and the metal shell, with at least part of the surface of the insulator being coated with a glaze layer comprising oxides, wherein the glaze layer comprises:
1 mol % or less, in terms of PbO, of a lead component,
35 to 80 mol % of a first component consisting of, based on the total glaze composition, 5 to 60 mol %, in terms of SiO2, of an Si component and 3 to 50 mol %, in terms of B2O3, of a B component,
5 to 60 mol % of a second component consisting of at least one of a Zn component and an alkaline earth metal R component (wherein R is at least one member selected from Ca, Sr, and Ba), the content of the Zn component and the R component being expressed in terms of ZnO and RO, respectively,
the total content of the first component and the second component being 65 to 98 mol %, and
2 to 15 mol % of at least one alkali metal component selected from an Na component, a K component, and an Li component in terms of Na2O, K2O or Li2O;
the insulator has an outwardly projecting portion on its periphery at the middle in the axial direction, the portion of the insulator which is in the rear of the projecting portion, the tip of the center electrode being taken as the front, (hereinafter referred to as the rear portion of the insulator) has a cylindrical shape around its base adjoining the projecting portion, and
the glaze layer is formed to cover the cylindrical shape with a thickness of 7 to 50 xcexcm.
In a vehicle engine, etc., a spark plug is connected to an electric part of the engine usually by means of a rubber cap. In order to secure anti-flashover properties, tightness between the insulator and the rubber cap is of importance. The present inventors have found it important to control the thickness of the glaze layer for obtaining a smooth glaze surface when a borosilicate or alkaline borosilicate glass type leadless glaze is used. They have ascertained that anti-flashover properties cannot be secured sufficiently without proper control on the glaze thickness around the base of the rear portion of the insulator because intimate contact with the rubber cap is especially required around this part. Hence, in the third aspect of the present invention, the thickness of the glaze layer covering the base of the rear portion of the insulator is limited within the above-specified range in addition to the limitation on the leadless glaze composition. According to the third aspect, an intimate fit of a rubber cap on the glaze surface can be achieved without impairing the insulating properties of the glaze layer, and excellent anti-flashover properties are ensured thereby.
If the thickness of the glaze layer on that part is smaller than 7 xcexcm, the leadless glaze as specified above encounters difficulty in forming a smooth glaze surface. As a result, the contact with a rubber cap will be defective, resulting in insufficient anti-flashover properties. With the thickness of the glaze layer exceeding 50 xcexcm, it would be difficult for the leadless glaze having the specified composition to secure insulating properties, which leads to insufficient anti-flashover properties. A preferred glaze layer thickness is 10 to 30 xcexcm.
The grounds for limiting the range of the content of each component constituting the glaze used in the third aspect of the present invention are as follows. If the Si component content in the glaze is less than 5 mol %, the glaze hardly vitrifies, resulting in a failure to form a uniform glaze layer. If, on the other hand, it exceeds 60 mol %, the glaze has too small a linear expansion coefficient, tending to suffer from defects such as cracking and peeling.
The B component content is 3 to 50 mol % as B2O3. If the B component content is less than 3 mol %, the softening point of the glaze increases to make firing difficult or impossible. If it is more than 50 mol %, a slurry of the glaze has insufficient durability (or water resistance) and, in addition, the resulting glaze layer tends to have such problems as devitrification, reduction in insulating properties, and missmatching of linear expansion coefficient with the substrate insulator.
Where the total content of the second component, which comprises a Zn component and/or an alkaline earth metal R component, is smaller than 5 mol %, it tends to be impossible to accomplish firing at a predetermined temperature on account of the elevated softening point, and the glaze will have a large linear expansion coefficient, easily resulting in defects such as crazing. Where the total content of the second component is more than 60 mol %, it tends to be impossible to accomplish firing at a predetermined temperature due to the elevated softening point, and the glaze layer may have insufficient insulating properties, leading to insufficient anti-flashover properties. The limitations imposed on the total content of the first and the second components and on the alkali metal component are based on the same reasons as described with reference to the first and the second aspects of the present invention.
The third aspect of the invention can be combined in the practice with the first and second aspects, provided that the glaze composition to be used be selected from the ranges common to the first and second aspects. Such a combination will provide a further improved glaze finish and enhances the effects of the third aspect.
If desired, the glaze layer according to the first, second, and third aspects of the invention can contain, in addition to the above-described essential components, 0.5 to 30 mol %, in total, of at least one of an Al component, a Ca component, and an Sr component, the content of the Al component being 0.5 to 10 mol % in terms of Al2O3, the content of the Ca component being 0.5 to 10 mol % in terms of CaO, and the content of the Sr component being 0.5 to 30 mol % in terms of SrO. An Al component is effective in suppressing devitrification of the glaze. A Ca component and an Sr component are contributory to improvement of insulating properties of the glaze layer. Amounts of the Al, Ca and Sr components lower than the respective lower limits produce scarce effects. When added in amounts greater than the respective upper limits, these components tend to increase the softening point of the glaze excessively, making firing difficult or impossible.
The present invention further provides in its fourth aspect a spark plug comprising a center electrode, a metal shell, and an alumina ceramic insulator disposed between said center electrode and said metal shell, with at least part of the surface of said insulator being coated with a glaze layer comprising oxides, wherein the glaze layer comprises:
1 mol % or less, in terms of PbO, of a lead component,
25 to 60 mol %, in terms of SiO2, of an Si component,
10 to 40 mol %, in terms of B2O3, of a B component,
0.5 to 9.5 mol %, in terms of ZnO, of a Zn component,
0.1 mol % or more, in terms of BaO, of a Ba component,
0.1 mol % or more, in terms of SrO, of an Sr component,
the total content of the Ba component and the Sr component being 5 to 25 mol % in terms of the respective oxides,
the total content of the Si component, the B component, the Zn component, the Ba component, and the Sr component being 60 to 98 mol % in terms of the respective oxides,
the content of the Ba component as BaO, taken as NBaO (mol %), and the content of the Sr component as SrO, taken as NSrO (mol %), satisfying the relationship: 4NBaOxe2x89xa6NSrO, and
2 to 15 mol % of at least one alkali metal component selected from an Na component, a K component, and an Li component in terms of Na2O, K2O or Li2O.
The glaze composition used in the fourth aspect and that of the first aspect have many of the essential components and their content ranges in common. The grounds for limiting the contents of the components described as to the first aspect substantially apply here. So the fourth aspect will be described only with reference to differences from the first aspect. The fourth aspect is characterized by positive addition of an Sr component. As previously noted, an Sr component is effective in improving mechanical strength, particularly impact strength, of a glazed insulator.
The present inventors have revealed that a reduction in Pb component in a glaze is apt to be accompanied with a relative reduction in mechanical strength, particularly impact resistance, of the glaze layer. As a result of further investigation, it has now been found that the impact resistance of a glaze layer can be improved markedly by increasing the content of an Sr component over a range that is decided in balance with other components. That is, the content of the Ba component as BaO, taken as NBaO (mol %), and the content of the Sr component as SrO, taken as NSrO (mol %), satisfy the relationship: 4NBaOxe2x89xa6NSrO. Barium and strontium belonging to the same group, a Ba component and an Sr component are similar in chemical characteristics. With respect to physical properties, particularly influences on the linear expansion coefficient of a glaze layer, however, an Sr component is less apt to increase the linear expansion coefficient. Therefore, an excessive increase of linear expansion coefficient of a glaze layer can be minimized by increasing the Sr component content relative to the Ba component content within such a range as satisfies the above-specified relationship. As a result, the difference in linear expansion coefficient between the glaze layer and the alumina ceramic insulator can be made smaller. It is considered that a tensile stress occurring in the glaze layer on glaze firing, which arises from a difference in linear expansion coefficient between the glaze layer and the substrate insulator, hardly remains in the glaze layer, resulting in marked improvement in impact strength.
The present invention further provides in its fifth aspect a spark plug comprising a center electrode, a metal shell, and an alumina ceramic insulator disposed between the center electrode and the metal shell, with at least part of the surface of the insulator being coated with a glaze layer comprising oxides, wherein the glaze layer comprises:
1 mol % or less, in terms of PbO, of a Pb component,
25 to 60 mol %, in terms of SiO2, of an Si component,
10 to 40 mol %, in terms of B2O3, of a B component,
0.5 to 9.5 mol %, in terms of ZnO, of a Zn component,
0.1 mol % or more, in terms of BaO, of a Ba component,
0.1 mol % or more, in terms of SrO, of an Sr component,
the total content of the Ba component and the Sr component being 5 to 25 mol % in terms of the respective oxides,
the total content of the Si component, the B component, the Zn component, the Ba component, and the Sr component being 60 to 98 mol % in terms of the respective oxides,
the content of the Zn component as ZnO, taken as NZnO (mol %), the content of the Ba component as BaO, taken as NBaO (mol %), and the content of the Sr component as SrO, taken as NSrO (mol %), totalizing 10 to 30 mol % and satisfying the relationship: NZnO/(NBaO+NSrO)xe2x89xa60.7, and
2 to 15 mol % of at least one alkali metal component selected from an Na component, a K component, and an Li component in terms of Na2O, K2O or Li2O.
Similarly to the fourth aspect, the fifth aspect of the invention aims at improvement of mechanical strength of a glazed insulator by positive addition of an Sr component. The total of NZnO, NBaO, and NSrO should be within the range of 10 to 30 mol %. If it is smaller than 10 mol %, the glaze tends to have an excessively elevated softening point, meeting difficulty in firing. If, on the other hand, that total is more than 30 mol %, the glaze layer tends to devitrify. A preferred total content (NZnO+NBaO+NSrO) is 15 to 25 mol %.
In addition, by limiting the value NZnO/(NBaO+NSrO) to 0.7 or smaller, an excessive increase of the linear expansion coefficient of the glaze layer can be suppressed to enhance the effect that the tensile stress due to the difference in linear expansion coefficient from the insulator hardly remains after firing. The impact resistance is further improved as a result.
The fourth and fifth aspects of the invention can be combined. The glaze of the fourth and fifth aspects can further comprise at least one of 0.5 to 10 mol %, in terms of Al2O3, of an Al component, 0.5 to 10 mol %, in terms of CaO, of a Ca component, and 0.5 to 10 mol %, in terms of MgO, of an Mg component, the total content of the Al component, the Ca component, and the Mg component being within a range of from 0.5 to 30 mol %. An Al component is effective in not only suppressing devitrification but further improving the impact resistance of the glaze layer. A Ca component and an Mg component are contributory to improvement of insulating properties of the glaze layer. Amounts of the Al, Ca and Mg components lower than the respective lower limits produce scarce effects. When added in amounts greater than the respective upper limits, these components tend to increase the softening point of the glaze excessively, making firing difficult or impossible.
In the light of factors causing fracture of the insulator of a spark plug, the present invention further provides in its sixth aspect a spark plug comprising a center electrode, a metal shell, and an alumina ceramic insulator disposed between the center electrode and the metal shell, with at least part of the surface of the insulator being coated with a glaze layer comprising oxides, wherein the glaze layer has a composition comprising:
1 mol % or less, in terms of PbO, of a lead component,
35 to 80 mol % of a first component consisting of, based on the total glaze composition, 5 to 60 mol %, in terms of SiO2, of an Si component and 3 to 50 mol %, in terms of B2O3, of a B component,
5 to 60 mol % of a second component consisting of at least one of a Zn component and an alkaline earth metal R component (wherein R is at least one member selected from Ca, Sr, and Ba), the content of the Zn component and the R component being expressed in terms of ZnO and RO, respectively,
the total content of the first component and the second component being 65 to 98 mol %, and
2 to 15 mol % of at least one alkali metal component selected from an Na component, a K component, and an Li component in terms of Na2O, K2O or Li2O;
the composition being adjusted so that the impact resistance of the insulator coated with the glaze layer may be such that the insulator has an impact-resistance angle of 35xc2x0 or greater as obtained by a pendulum impact test in which,
(1) the spark plug is vertically fixed to a mount by means of the metal shell with the sparking tip thereof, taken as the front, inside the mount and the rear portion of the insulator projecting upright from the rear end of the metal shell,
(2) a pendulum having a 330 mm long arm and a steel striker weighing 1.13 kg at the tip thereof is set so as to be allowed to swing on its support (shoulder) that is positioned above the rear end of the projecting insulator in the axial direction of the insulator at such a height that the striker strikes 1 mm vertically below the rear end of the insulator,
(3) the pendulum is allowed to swing through a prescribed angle xcex8 from the vertical repeatedly while increasing the angle xcex8 stepwise by 2xc2x0 until the insulator is broken, and
(4) the critical angle xcex8 at which the insulator is broken is taken as an impact-resistance angle.
In recent internal combustion engines showing a remarkable increase of power output, the spark plugs are exposed to considerable vibrations and shocks in operation, and their fracture is of great concern. When the spark plug is fitted to a cylinder head particularly by using a power tool such as an impact wrench, it might be fractured under excess clamping torque. Hence, the composition and thickness of the glaze layer are adjusted so that the glazed insulator may have the above-identified impact-resistance angle of 35xc2x0 or greater, whereby the insulator can effectively be protected from vibrations or shocks and prevented from fracture.
The glaze used in the sixth aspect can have the composition specified in the fourth or fifth aspect. In other words, the sixth aspect can be practiced in combination with the fourth or fifth aspect of the invention. The structural feature of the third aspect of the invention can be introduced into the insulator according to the fourth to sixth aspects. That is, the insulator can have an outwardly projecting portion on its periphery at the middle in the axial direction, and the rear portion of the insulator (i.e., the portion in the rear of the projecting portion as previously defined) has a cylindrical periphery around its base adjoining the projecting portion. The glaze layer is formed to cover the cylindrical periphery around the base with a thickness of 7 to 50 xcexcm. When this structure is adopted, not only are anti-flashover properties improved as stated above but the impact resistance of the glazed insulator is further improved. When the glaze layer on that part is thinner than 7 xcexcm, the anti-flashover properties tend to be insufficient, and such a thin glaze layer may fail to secure its own strength or to produce sufficient effects in hiding the surface defects of the insulator, resulting in insufficient impact resistance. If the thickness exceeds 50 xcexcm, it would be difficult to secure insulating properties with the leadless glaze as specified, resulting in insufficient anti-flashover properties. Moreover, the residual stress after firing, which depends on the linear expansion coefficient and thickness of the glaze layer, tends to be too high, which can result in insufficient impact resistance. A still preferred glaze layer thickness is 10 to 30 xcexcm.
Supplementary particulars which are common among the first through sixth aspects of the present invention will be described.
One or more of an Mo component, an Fe component, a W component, an Ni component, a Co component, and an Mn component can be added as auxiliary components to the glaze composition of the invention in a total amount of from 0.5 to 5 mol % in terms of the respective oxides, MoO3, Fe2O3, ZrO2, WO3, Ni3O4, Co3O4, and MnO2 to provide, with more ease, a glaze composition which exhibits markedly improved flowability on firing and therefore can be fired at a relatively low temperature to form a glaze layer having excellent insulating properties and a smooth surface.
Where the total content, in terms of the respective oxides, of at least one component of Mo, W, Ni, Co, Fe or Mn (hereinafter referred to as a flowability-improving transition metal component) is less than 0.5 mol %, the effect on flowability improvement for obtaining a smooth glaze layer is insubstantial. Where it exceeds 5 mol %, the softening point of the glaze tends to rise to make firing difficult or impossible.
When the content of the flowability-improving transition metal component is too high, there arises a problem that the glaze layer tends to suffer from unintended coloration. Pieces of information such as letters showing the name of a manufacturer, a figure, a lot number, etc. are often printed on the insulator in a colored glaze for labeling. If the background glaze layer assumes a noticeable color, the print would be invisible. Still another practical problem that can arise is that the color change of a glaze layer ascribed to an alteration to the composition may be taken by purchasers as a groundless alteration from xe2x80x9cthexe2x80x9d accustomed color and may lose their full acceptance.
Of these flowability-improving transition metals, Mo and Fe are the most effective, and W is the next in improving flowability of a molten glaze. It is acceptable that the flowability-improving transition metal component consists solely of Mo, Fe or W. For enhancing the flowability improving effect, it is preferred for the flowability-improving transition metal component to comprise at least 50 mol % of an Mo component. While the Fe source of the glaze raw material may be in the form of either an Fe(II) ion (as in, e.g., FeO) or an Fe(III) ion (as in, e.g., Fe2O3), the Fe component content in the resulting glaze layer is represented in terms of Fe2O3 irrespective of the valency of Fe ions.
The glaze composition can further contain one or more of a Zr component, a Ti component, an Hf component, an Mg component, a Bi component, an Sn component, an Sb component, and a P component as auxiliary components in a total amount of 0.5 to 5 mol % in terms of the respective oxides, ZrO2, TiO2, HfO2, MgO, Bi2O3, SnO2, Sb2O5, and P2O5. These auxiliary components can be added as external additives according to necessity, or they are unavoidably incorporated as impurities (or contaminants) originated in raw materials (or a clay mineral hereinafter described, which is added in the preparation of a glaze slurry) or refractories used in a melting step.
These auxiliary components are appropriately added according to the purpose, for example, of controlling the softening point (Bi2O3, ZrO2, TiO2 or HfO2 can serve for this), of improving insulating properties (ZrO2 or MgO can serve for this), and adjusting the color tone. Addition of a Ti component, a Zr component or an Hf component brings about improvement on durability of the glaze. A Zr component or an Hf component is more effective in improving durability than a Ti component. For a glaze to have xe2x80x9csatisfactory durabilityxe2x80x9d means that a slurry of a glaze powder in a medium, such as water, does not increase its viscosity due to dissolution of a component in water when left to stand for a long time. With a glaze slurry having satisfactory durability, it would be easy to coat the insulator to a proper thickness with reduced thickness variation. It follows that the glaze layer formed on firing could have a proper thickness as intended with reduced thickness variation. An Sb component or a Bi component is effective in improving the flowability on firing to suppress seed or bubble formation in the resulting glaze layer and also to entrap foreign matter adhered to the surface into the flow thereby preventing it from becoming abnormal projections.
The present invention further provides in its seventh aspect a spark plug comprising a center electrode, a metal shell, and an alumina ceramic insulator disposed between the center electrode and the metal shell, at least part of the surface of the insulator being coated with a glaze layer comprising oxides, wherein the glaze layer comprises:
1 mol % or less, in terms of PbO, of a lead component,
35 to 80 mol % of a first component consisting of, based on the total glaze, 5 to 60 mol %, in terms of SiO2, of a silicon component and 3 to 50 mol %, in terms of B2O3, of a boron component,
5 to 60 mol % of a second component consisting of at least one of a zinc component and an alkaline earth metal R component (wherein R is at least one member selected from Ca, Sr, and Ba), the content of the zinc component and the R component being expressed in terms of ZnO and RO, respectively,
the total content of the first component and the second component being 60 to 98 mol %,
2 to 15 mol % of at least one alkali metal component selected from a sodium component, a potassium component, and a lithium component in terms of Na2O, K2O or Li2O, and
0.5 to 5 mol % of at least one transition metal component selected from a molybdenum component, a tungsten component, a nickel component, a cobalt component, an iron component, and a manganese component, the content of the at least one transition metal component being expressed in terms of MoO3, WO3, Ni3O4, Co3O4, Fe2O3 and MnO2.
From the environmental consideration it is a premise of the present invention that the Pb component content in the glaze be 1.0 mol % or less in term of PbO. A glaze with its Pb component content reduced to this level will hereinafter be referred to as a leadless glaze. When a Pb component is present in a glaze in the form of an ion of lower valency (e.g., Pb2+), it can be oxidized to an ion of higher valency (e.g., Pb3+) by a corona discharge from the glaze layer surface, etc.
If this happens, the insulating properties of the glaze layer are reduced, which can result in a flashover phenomenon. From this viewpoint, too, the limited Pb content is beneficial. A preferred Pb content is 0.1 mol % or less. It is the most preferred for the glaze to contain substantially no Pb, except a trace amount of lead unavoidably incorporated together with raw materials.
The glaze used in the seventh aspect of the present invention has a specifically designed composition for securing insulating properties, optimizing the firing temperature, and improving the firing finish while reducing the Pb content. A Pb component in a conventional glaze has played an important role in softening point adjustment. That is, a Pb component serves to lower the softening point of a glaze moderately to secure flowability in application. In a leadless glaze, it is a B component (B2O3) and an alkali metal component that take part in softening point adjustment. The present inventors have found that there is a specific range of a B component content relative to an Si component content that is suited to improve firing finish. Within this specific B content range, they have also found that addition of at least one of Mo, W, Ni, Co, Fe, and Mn in a specific amount provides a glaze composition that exhibits markedly improved flowability and therefore can be fired at a relatively low temperature to form a glaze layer having excellent insulating properties and a smooth glazed surface. The present invention has been reached based on these findings.
The grounds for limiting the range of the content of each component constituting the glaze are as follows. Where the total content, in terms of oxides, of at least one component of Mo, W, Ni, Co, Fe or Mn (hereinafter referred to as an essential transition metal component) is less than 0.5 mol %, the effect on flowability improvement is insufficient for obtaining a smooth glaze layer. Where it exceeds 5 mol %, an excessively elevated softening point tends to make firing difficult or impossible.
When the content of the essential transition metal component is too high, there arises another problem that the glaze tends to suffer from unintended coloration. Pieces of information such as letters showing the name of a manufacturer, a figure, a lot number, etc. are often printed on the insulator in a colored glaze for labeling. If the background glaze layer assumes a noticeable color, the print would be invisible. Still another practical problem that can arise is that the color change of a glaze layer ascribed to an alteration to the composition may be taken by purchasers as a groundless alteration from xe2x80x9cthexe2x80x9d accustomed color and may lose their full acceptance.
The insulator used in the invention, which is a substrate to be glazed, is made up of white alumina ceramic. In order to prevent or minimize the above-described inconveniences due to coloration, it is desirable to control the glaze composition, for example, the content of the essential transition metal component so that the glaze layer formed on such a white insulator can have a chroma Cs of 0 to 6 and a value Vs (lightness) of 7.5 to 10. A hue with a chroma exceeding 6 is noticeably perceived by the naked eye, and a hue with a value smaller than 7.5 appears grayish or blacky. A glaze layer whose color is out of either one of the above chroma and value ranges cannot dispel the impression that it is obviously colored. It is desirable that the chroma Cs be 0 to 2, particularly 0 to 1, and that the value Vs be 8 to 10, particularly 9 to 10. In the present invention the value Vs and the chroma Cs are measured in accordance with the methods specified in JIS Z8722 xe2x80x9cMethods of Colour Measurement/4. Spectrophotometric colorimetry/4.3 Method of Measuring Reflecting Objectsxe2x80x9d. In a simpler manner, the value and chroma can also be obtained by visual color judgement using standard color chips prepared according to JIS Z8721.
Of the essential transition metals, Mo is the most effective, and W is the next in improving flowability of a molten glaze. It is acceptable that the essential transition metal component consists solely of Mo or W. For enhancing the flowability improving effect, it is preferred for the essential transition metal component to comprise at least 50 mol % of an Mo component.
If the Si component content in the glaze is less than 5 mol %, the glaze hardly vitrifies, resulting in a failure to form a uniform glaze layer. If, on the other hand, it exceeds 60 mol %, the glaze has too small a linear expansion coefficient, tending to suffer from defects such as cracking and peeling. A preferred Si component content is 15 to 60 mol %.
If the B component content is less than 3 mol %, the softening point of the glaze increases to make firing difficult or impossible. If it is more than 50 mol %, the glaze layer has insufficient durability (or water resistance) and, in addition, tends to have such problems as devitrification, reduction in insulating properties, and missmatching of linear expansion coefficient with the substrate insulator. A preferred B component content is 10 to 50 mol %. It is preferred that the ratio of the Si component content, taken as NSiO2 (mol %), to the B component content, taken as NB2O3 (mol %), i.e., NSiO2/NB2O3, be in the range of from 0.5 to 1.5. When this ratio is smaller than 0.5, inconveniences such as devitrification, reduction in insulating properties, and missmatching of linear expansion coefficient with the substrate, can result. With the ratio exceeding 1.5, the glaze has too small a linear expansion coefficient, tending to suffer from defects such as cracking and peeling.
In a preferred embodiment, the first component (i.e., the Si component plus the B component) comprises 15 to 29.5 mol % of the Si component as SiO2 and 25 to 50 mol % of the B component as B2O3 based on the total glaze composition. In this embodiment, the glaze has a moderately lowered softening point to provide sufficiently flowable molten glass, which will make a glazed surface with good finish even in short-time firing. Glaze defects, such as crawling, crimping, parting, and pinholes, are also suppressed effectively.
Where the total content of the second component, which comprises a Zn component and/or an alkaline earth metal R component, is smaller than 5 mol %, it tends to be impossible to accomplish firing at a predetermined temperature on account of the elevated softening point, and the resulting glaze layer tends to have reduced anti-flashover properties due to insufficient insulating properties. Where the total content of the second component is more than 60 mol %, it tends to be impossible to accomplish firing at a predetermined temperature due to the elevated softening point, and the glaze will have too large a linear expansion coefficient, which may lead to such defects as crazing.
The total content of the first and the second components is from 60 to 98 mol %. If that total content exceeds 98 mol %, the glaze tends to have too high a softening point to fire. If the total content is smaller than 60 mol %, it is difficult to adjust the softening point and the linear expansion coefficient while securing insulating properties. A preferred total content of the first and the second components is 70 to 95 mol %.
Of the components making up the second component, a Zn component and a Ba component are particularly contributory to improvement in insulating properties and also effective in improving durability and strength. It is preferred to use the Zn component and the Ba component in amounts of 0.5 to 25 mol % as ZnO and 5 to 25 mol % as BaO. With a Zn component content of smaller than 0.5 mol %, the glaze may have too small a linear expansion coefficient, tending to suffer from defects, such as cracking and peeling. Since a Zn component is also effective on improvement of insulating properties, its shortage can result in insufficient insulation. Where the content of the Zn component is more than 25 mol %, devitrification can occur to make the glaze layer opaque. When the Ba component content is less than 5 mol %, the glaze has reduced insulating properties, tending to have insufficient anti-flashover properties. If the Ba component exceeds 25 mol %, the softening point tends to become too high to carry out firing.
The alkali metal component serves to lower the softening point of a glaze. If the alkali metal component content is less than 2 mol %, the glaze will have an increased softening point, which tends to be too high to conduct firing. If it exceeds 15 mol %, the glaze tends to have reduced insulating properties, resulting in insufficient anti-flashover properties. A preferred alkali metal component content is 3 to 10 mol %. The ratio of the alkali metal component, taken as NQ2O (mol %), to the B component content (NB2O3; mol %) (NQ2O/NB2O3) is preferably 0.1 to 0.25. When the ratio is smaller than 0.1, the softening point of the glaze may tend to be too high for firing. If the ratio exceeds 0.25, the glaze tends to have reduced insulating properties, which may lead to insufficient anti-flashover properties.
It is preferred to use a combination of at least two kinds selected from an Na component, a K component, and an Li component as the alkali metal component, which is effective in suppressing reduction in insulating properties. This means that the alkali metal content is allowed to increase while minimizing reduction in insulating properties. As a result, the two objectsxe2x80x94to secure anti-flashover properties and to lower the firing temperaturexe2x80x94can be accomplished at a time. It is possible to add as a third component other alkali metal component(s) in such an amount that does not impair the above-described effect of combined alkali metal components in suppressing electrical conductivity. In order to minimize the reduction in insulating properties, the amount of each alkali metal component to be added is desirably 5 mol % or less.
If desired, the glaze of the seventh aspect of the present invention can contain the following components in addition to the above-described essential components.
The glaze can contain one or more of 0.5 to 10 mol %, in terms of Al2O3, of an Al component, 0.5 to 10 mol %, in terms of CaO, of a Ca component, and 0.5 to 30 mol %, in terms of SrO, of one an Sr component in a total content (Al+Ca+Sr) of 0.5 to 30 mol %. An Al component is effective in suppressing devitrification of the glaze. A Ca component and an Sr component are contributory to improvement of insulating properties of the glaze. Amounts of the Al, Ca and Sr components lower than the respective lower limits produce scarce effects. When added in amounts greater than the respective upper limits, these components tend to increase the softening point of the glaze excessively, making firing difficult or impossible.
The glaze can further contain one or more of an Fe component, a Zr component, a Ti component, an Mg component, a Bi component, an Sn component, an Sb component, and a P component as auxiliary components in a total amount up to 5 mol % in terms of the respective oxides, Fe2O3, ZrO2, TiO2, MgO, Bi2O3, SnO2, Sb2O5, and P2O5. These auxiliary components can be added as external additives according to necessity or unavoidably incorporated as impurities (or contaminants) originated in raw materials (or a clay mineral hereinafter described, which is added in the preparation of a glaze slurry) or refractories used in a melting step. While the Fe source of the glaze raw material may be in the form of either an Fe(II) ion (as in, e.g., FeO) or an Fe(III) ion (as in, e.g., Fe2O3), the Fe component content in the resulting glaze layer is represented in terms of Fe2O3 irrespective of the valency of Fe ions. These auxiliary components are appropriately added according to the purpose, for example, of controlling the softening point (Bi2O3, ZrO2 or TiO2 can serve for this), of improving insulating properties (ZrO2 or MgO can serve for this), and adjusting the color tone. Addition of a Ti component or a Zr component brings about improvement on durability or chemical resistance of the glaze layer and suppresses the alkali metal component from dissolving out of the glaze thereby making contribution to improvement of dielectric strength. In particular, a Zr component is more effective in improving the chemical resistance than a Ti component. For a glaze composition or a formed glaze layer to have xe2x80x9csatisfactory durabilityxe2x80x9d means not only that a component hardly dissolves into water from a formed glaze layer but that an aqueous slurry of a glaze frit does not increase its viscosity due to elution of a component in water when left to stand for a long time. An Sb component is effective to suppress seed or bubble formation in a glaze layer.
While each of the aforementioned components in the glaze layer of the first to seventh aspects of the present invention exists in an oxide form, it is often impossible to identify the state of existence because, for one thing, they form an amorphous glass phase. Such cases are also included under the scope of the present invention as long as the contents of the components in terms of the respective oxides fall within the respective ranges as specified.
The content of each component constituting a glaze layer formed on the insulator can be determined by known microanalyses, such as electronic probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS). In carrying out EPMA, for instance, characteristic X-rays can be measured by either wavelength dispersive analysis or energy dispersive analysis. The composition can also be identified by peeling the glaze layer off the insulator and subjecting the peel to chemical analysis or gas analysis.
The spark plug according to the first to seventh aspects of the present invention can be constructed of an insulator having a through-hole through the middle, a center electrode inserted in the through-hole, and a co-axial terminal which is an integral part of the center electrode or which is a separate part and connected to the center electrode via an electrically conductive binder layer. The insulation resistance of the spark plug is measured by applying voltage between the terminal and the metal shell through the insulator while maintaining the whole spark plug at about 500xc2x0 C. In order to secure dielectric strength in high temperature and to prevent a flashover from happening, it is preferred for the spark plug to have an insulation resistance of 200 MQ or more.
An example of a system for insulation resistance measurement is shown in FIG. 8, in which a constant DC voltage power source (e.g., powder voltage: 1000 V) is connected to a terminal 13 of a spark plug 100, and a metal shell 1 is grounded. Voltage is applied while the spark plug 100 is being heated to 500xc2x0 C. in a heating oven. In measuring a current Im by use of a resistor for current measurement (resistance: Rm) at a voltage VS, an insulation resistance Rx is obtained as [(VS/Im)xe2x88x92Rm]. In FIG. 8 the current Im is obtained from an output of a differential amplifier which amplifies the voltage difference between the terminals of the resistor.
The insulator is made of an alumina-based insulating material containing 85 to 98 mol %, in terms of Al2O3, of an Al component. It is preferred for the glaze to have an average linear expansion coefficient of 50xc3x9710xe2x88x927/xc2x0 C. to 85xc3x9710xe2x88x927/xc2x0 C. in a temperature range of from 20 to 350xc2x0 C. Where the linear expansion coefficient is smaller than the lower limit, the glaze layer is liable to suffer from detects such as cracking and peeling. Where the linear expansion coefficient is greater than the upper limit, the glaze layer is apt to suffer from such defects as crazing. A still preferred linear expansion coefficient of the glaze ranges from 60xc3x9710xe2x88x927/xc2x0 C. to 80xc3x9710xe2x88x927/xc2x0 C.
The linear expansion coefficient of a glaze can be estimated from the value obtained with a known dilatometer on a specimen cut out of a glass block prepared by compounding and melting raw materials so as to give substantially the same composition as a glaze layer. The linear expansion coefficient of a glaze layer as formed on an insulator can be measured with, e.g., a laser interference meter or an atomic force microscope.
The spark plug of the first to seventh aspects of the present invention can be produced by, for example, a process comprising:
a step of preparing a glaze powder in which raw material powders are mixed in a predetermined ratio, the mixture is melted at 1000 to 1500xc2x0 C. and quenched for vitrification, and grinding the glass into powder (a frit),
a step of depositing the glaze powder on the surface of an insulator to form a glaze powder deposit, and
a step of firing in which the insulator is fired to bake the glaze powder deposit onto the insulator surface to form a glaze layer.
The powdered raw material of each component includes not only an oxide or a complex oxide but other various inorganic materials capable of being converted to a corresponding oxide on heating and melting, such as a hydroxide, a carbonate, a chloride, a sulfate, a nitrate, and a phosphate. The quenching can be carried out by pouring the melt in water or atomizing the melt onto a chill roll to obtain flakes.
The glaze powder can be formulated into a slurry in water or a solvent. The slurry is applied to the insulator and dried to form a coating layer of the deposited glaze powder. The glaze slurry is conveniently applied onto the insulator by spraying from a spray nozzle to deposit a glaze powder to a uniform thickness with ease of thickness control.
The glaze slurry can contain an adequate amount of a clay mineral or an organic binder to improve the shape retention of the glaze powder deposited layer. Useful clay minerals include those comprising aluminosilicate hydrates, such as allophane, imogolite, hisingerite, smectite, kaolinite, halloysite, montmorillonite, vermiculite, and dolomite, which may be either natural or synthetic, and mixtures thereof. In relation to the oxide components of the glaze composition, clay minerals containing one or more of Fe2O3, TiO2, CaO, MgO, Na2O, and K2O in addition to SiO2 and Al2O3 can be used.
The spark plug according to the first to seventh aspects of the present invention is constructed of an insulator having a through-hole piercing in the axial direction, a terminal fitted into one end of the through-hole, and a center electrode fitted into the other end. The terminal and the center electrode are electrically connected via an electrically conductive sintered body comprising glass and a conductive material (e.g., conductive glass seal or a resistor). The spark plug having such a structure can be made by a process including the following steps.
An assembly step: a step of assembling a structure comprising an insulator having a through-hole, a terminal fitted into one end of the through-hole, a center electrode fitted into the other end, and a green body formed between the terminal and the center electrode, the green body comprising a glass powder and a conductive material powder.
A firing step: a step of heating the structure having thereon a glaze powder deposited layer at a temperature of 800 to 950xc2x0 C. to bake the glaze powder on the insulator to form a glaze layer and, at the same time, softening the glass powder in the green body.
A pressing step: a step of relatively bringing the center electrode and the terminal close within the through-hole of the heated structure thereby pressing the green body between the two members into an electrically conductive sintered body.
The conductive sintered body establishes an electrical connection between the terminal and the center electrode and seals the gap between the inner surface of the through-hole and the terminal and the center electrode. Therefore, the firing step also serves as a glass sealing step. The above-described process is efficient in that glass sealing and glaze firing are performed simultaneously. Further, since the above-described glaze composition allows the firing temperature to be reduced to 800 to 950xc2x0 C., the center electrode and the terminal hardly suffer from oxidative damage so that the yield is improved.
The softening point of the glaze is preferably adjusted within a range of from 600 to 700xc2x0 C. When the softening point is higher than 700xc2x0 C., a firing temperature above 950xc2x0 C. would be required to carry out both firing and glass sealing, which may accelerate oxidation of the center electrode and the terminal. When the softening point is lower than 600xc2x0 C., the firing temperature should be set lower than 800xc2x0 C., in which case the glass used in the conductive sintered body must have a low softening point in order to secure satisfactory glass seal. It follows that the glass in the conductive sintered body is liable to denaturation in long-term use of the spark plug in a relatively high temperature environment. Where, for example, the conductive sintered body comprises a resistor, such glass denaturation tends to result in deterioration of the performance, such as a life under load.
The term xe2x80x9csoftening pointxe2x80x9d of a glaze as used herein is a value measured by differential thermal analysis (DTA) on a glaze layer peeled off the insulator. It is obtained as a temperature of the peak appearing next to the first andothermic peak which is indicative of a sag point, i.e., the second endothermic peak temperature of a DTA curve. The softening point of a glaze can also be estimated from the value obtained with a glass sample which is prepared by compounding raw materials so as to give substantially the same composition as the glaze layer under analysis, melting the composition, and quenching. The composition to be used can be calculated on an oxide basis from the data obtained from the glaze layer to be analyzed.